1. Field of the Invention
The present invention relates to a self-calibrating oversampling electromechanical modulator and to a self-calibration method.
2. Description of the Related Art
As is known, the use of micro-electromechanical-system (MEMS) sensors with differential capacitive unbalance has been proposed for building, for example, linear or rotational accelerometers and pressure sensors.
In particular, MEMS sensors of the above-mentioned type comprise a fixed body (stator) and a mobile mass, which are generally of an appropriately doped semiconductor material, are connected together by elastic-suspension elements (springs) and are constrained in such a way that the mobile mass has, with respect to the stator, predetermined degrees of freedom, which are translational and/or rotational. In addition, the stator and the mobile mass have a plurality of fixed arms and of mobile arms, respectively, in a comb-finger arrangement. In practice, each mobile arm is arranged between a pair of fixed arms, so as to form a pair of capacitors which have a common terminal and a capacitance that depends upon the relative positions of the arms, namely upon the position of the mobile mass with respect to the stator (sensing capacitance). The fixed arms are then connected to external sensing terminals. When a sensor is excited, its mobile mass is displaced and there is an unbalance between the capacitances of the capacitors, which can be detected at the sensing terminals.
In addition, MEMS sensors are equipped with actuation capacitors, which are provided between the stator and the mobile mass and are connected to external actuation terminals. When a voltage is supplied on said actuation terminals, between the plates of the actuation capacitors an electrostatic actuation force is exerted (in all cases of an attractive type), which displaces the mobile mass with respect to the stator. The actuation terminals may even coincide with the sensing terminals.
MEMS sensors are normally associated to electronic read and control components, with which they form oversampling electromechanical modulators.
For greater clarity, reference may be made to FIG. 1, which shows an oversampling electromechanical modulator 1 comprising a MEMS sensor 2, for example a linear-type accelerometer, a charge integrator 3, a one-bit quantizer 4, and a feedback actuator 5, which are connected together so as to form a control loop. In greater detail, the MEMS sensor 2, the charge integrator 3 and the quantizer 4 form the forward path of the control loop, while the feedback actuator 5, which is connected between an output of the quantizer 4 and an actuation input 2a of the MEMS sensor 2, forms the feedback line.
The MEMS sensor 2 is connected to the charge integrator 3, which, in a sensing step, detects the capacitive unbalance of the sensor 2 and supplies, on an output—which is connected to an input of the quantizer 4, an analog signal VM correlated to said capacitive unbalance. The quantizer 4 generates at its output a bitstream BS, in which each bit represents the sign of the analog signal VM at a respective sampling instant.
The feedback actuator 5 receives at input the bitstream BS and, in an actuation step following upon the sensing step, supplies to the actuation input 2a of the MEMS sensor 2 a feedback-actuation voltage VFB for counteracting the displacement of the mobile mass of the MEMS sensor 2 and bringing the mobile mass back into the resting position.
In an ideal MEMS sensor, when no external stress are present and no voltages are applied to the actuation terminals, the mobile arms should be exactly in an intermediate position between the respective fixed arms that are arranged facing them, and the capacitances should be balanced. This means that in an ideal electromechanical modulator the mobile mass of the MEMS sensor should oscillate about the nominal resting position, and the bitstream BS should have a zero average (namely, the bitstream BS should be formed by a sequence of bits having alternating logic values, such as +1 −1 +1 −1, etc.).
In actual fact, notwithstanding the extremely high precision of the micromachining techniques used for building MEMS sensors, it is unavoidable that the mobile mass is affected by a position offset; consequently, also in resting conditions the mobile arms are not equidistant from the fixed arms. As a result, MEMS sensors have an intrinsic capacitive unbalance which, in an electromechanical modulator, causes an offset of the bitstream BS (in practice, the average of the bitstream BS is not zero).
At present, in order to correct the offset of electromechanical modulators, an in-factory calibration process is carried out, which involves various steps and which will be briefly described with reference to FIG. 2. In addition to illustrating the electromechanical modulator 1, FIG. 2 also shows a measurement-interface circuit 7 and a calibration circuit 8. In particular, the calibration circuit 8 is programmable and supplies a calibration voltage VCAL to a calibration terminal 2b of the MEMS sensor 2 in order to exert an electrostatic force on the mobile mass of the MEMS sensor 2 itself.
First of all, the electromechanical modulator 1 is set in a quiescent state, in which the MEMS sensor 2 does not undergo any stress, and the feedback loop is opened by disconnecting the feedback actuator 5 from the actuation terminal 2a of the MEMS sensor 2.
Next, the measurement-interface circuit 7 is connected to the input of the quantizer 4 and detects the value of the analog signal VM, which, in the conditions described, is due exclusively to the position offset of the mobile mass of the MEMS sensor 2. In particular, the measurement-interface circuit 7 generates an offset signal VOFF correlated to the analog signal VM. 
Next, the calibration circuit 8 is programmed by causing the calibration voltage VCAL to vary until the offset signal VOFF is minimized and the mobile mass of the MEMS sensor 2 is brought back into the proximity of the nominal resting position.
Subsequently, if the sensing capacitances present between the mobile mass and the stator of the MEMS sensor 2 are unbalanced, the calibration is completed by connecting one or more calibration capacitors 9 in parallel to the smaller sensing capacitance.
The devices according to the prior art have some drawbacks. In the first place, calibration can be performed only in the factory, and consequently it cannot be ensured that the precision will remain unaltered over time. In fact, the mechanical properties of a MEMS sensor, especially as regards the elastic-suspension elements, are affected by environmental conditions (for instance, by the temperature) and in any case vary on account of the ageing of the MEMS sensor itself. In practice, the initial calibration is lost and an offset arises again.
In addition, MEMS sensors are extremely sensitive and are able to detect even minimal vibrations. Consequently, it is very difficult to create a condition of effective absence of stress in which a precise calibration can be performed.